This invention relates to a method and an apparatus for meniscus coating at least one surface of steel strip with molten metal. More particularly, the invention relates to moving at least one of the strip surfaces transversely past a departure lip of a horizontally disposed coating tray containing the molten metal. The strip surface is wetted by meniscus contact with the molten metal flowing over the departure lip and onto the passing strip.
It has been known for many years the corrosion resistance of steel strip could be enhanced by immersion into a bath of molten metal. Product quality in an immersion process is inconsistent because of changes in the surface condition of the pot rolls in the bath. This surface condition change is caused by erosion to the roll surface and build up of iron intermetallic particles on the roll surface. This pot roll surface condition may mark the strip surface. The strip surface also can be scratched if the strip drifts across the pot roll surface. A further product quality problem associated with immersion coating is nonuniform coating thickness because of pass line instability and poor strip shape.
Another problem associated with immersion coating is the requirement for a large molten metal reservoir. The large pot size requires considerable capital expense during initial installation, requires significant maintenance expense and requires considerable operating expense for the thermal input necessary to maintain the bath temperature.
A further problem associated with immersion coating relates to scheduling a coating line, particularly in the steel industry. Scheduling a coating line according to strip thickness and width is important for producing high quality material. Thin strip is easily damaged and preferably coated using fresh pot rolls. Because pot roll build up frequently occurs at those portions of the pot roll corresponding to strip edges, wider strip normally is not scheduled to follow narrower strip. This unpredictable service life of coating pot equipment results in unscheduled coating line stoppages.
Scheduled production runs normally are for a long duration with steel strip receiving the same coating type with only gradual decreasing width changes being permitted. This may require maintaining an excess amount of steel inventory for extended periods of time because strip requiring a coating metal type or a width not corresponding to the current production schedule can not be scheduled. This not only increases costs for the manufacturer but also for the customer.
More recently, techniques have been developed to coat one or both sides of steel strip with molten metals using a meniscus. U.S. Pat. No. 4,557,953 discloses horizontal meniscus coating one side of steel strip. A cleaned strip is passed from a snout chamber to a large coating pot containing molten metal. Deflection rolls are used to pass the strip sufficiently close to the molten metal surface so that molten metal wets the lower surface of the strip. Molten metal is withdrawn from the pot onto the surface of the strip. U.S. Pat. No. 4,529,628 discloses vertical meniscus coating one side of a steel strip. A coating device is provided to include a melting furnace having a lateral distribution conduit whose outlet communicates with an externally open release aperture serving to distribute molten metal over the entire width of a vertically traveling strip. Pressurized molten metal is forced through the release aperture and flows downwardly by gravity into a gap formed between the aperture and the strip. Japanese patent application 61-207556 also discloses vertical meniscus coating one side of steel strip. A tank containing molten metal includes a plating nozzle for positioning close to a surface of a vertically traveling strip. The level of the molten metal is maintained in the tank at a level above the elevation of the nozzle using a head pressure of 10-30 mm so that the molten metal can be withdrawn from the nozzle onto the strip surface.
U.S. Pat. No. 2,914,423 discloses coating a metal strand such as steel wire or strip. A molten metal reservoir includes a conically shaped extension with the strand being passed vertically up through an orifice in the center of the extension.
Nevertheless, there remains a need for a high speed process for coating one or both surfaces of steel strip with molten metal that can eliminate product quality problems such as nonuniform coating thickness and poor strip shape. There also remains a need for a high speed process providing uninterrupted coating line operation when it becomes necessary to change the molten metal type, strip width, the number of surfaces of the strip to be coated or when coating both surfaces of the strip with different types of molten metal. There also is a need for a high speed coating process where the coating bath does not include iron intermetallics. There is also a need for a high speed coating process where the strip surface is not damaged by a pot roll. Furthermore, there remains a need for a high speed process that does not require pressurized delivery of molten metal onto the strip surface or a large reservoir for the molten metal.